In certain electronic devices, input/output (I/O) buffers have multiple power supplies, possibly providing different voltage levels. During initial power-on of such an electronic device, because different power supplies may be driven high at different start times and at different ramp rates, it may be important to monitor the relative relationship between the levels of the multiple power supplies, e.g., in order to minimize power dissipation, bus contention in hot-socket applications, and noise due to oscillations of the bias networks. The outputs of such monitoring circuits are used as control signals for the I/O buffers as well as for other sections of the electronic device to intermittently control the state of the circuitry to meet the device's specifications. Noise on the power supplies and the different ramp rates at which they rise can cause the monitors to oscillate around their bias switch points, resulting in noise in the outputs of the monitoring circuits, increased power dissipation within the monitoring circuits, and indeterminate states of the circuits being controlled.
Prior-art techniques to monitor the relative relationship between two power supply levels use a single bias-point monitor that can cause oscillations if the power supplies have noise or are unstable. With one bias monitor, the monitoring circuit can oscillate, even with very little input noise, if the supply voltages are close to the switch bias point. Biasing such a monitor to default to one of the power supplies may protect the circuit from oscillating, but it may also cause the circuit to select the wrong power supply under certain conditions, which can lead to possible damage, excessive power draw, and bus contention.